The invention relates to a method for detecting and sorting glass in a scrap glass stream, preferably a stream of broken glass, wherein light rays emitted from a radiation source traverse the scrap glass stream and impinge on a detector unit. These light rays are processed by an evaluation and control unit in data communication therewith, the evaluation and control unit connected to the detector unit activating, as a function thereof, a removal apparatus such as a blowing nozzle. This nozzle is disposed downstream of the detector unit. This removal apparatus is for singling out undesired matter entrained in the scrap glass stream and diverting it to a predetermined location.
The recycling of scrap and broken glass together with an organized collection and selection method has been successfully practiced for quite a long time and has been capable of significantly reducing the energy expenditure in the industrial production of glass utensils. The known difficulty that the consumers collecting scrap glass are not very careful in separating the material with respect to color and to materials other than glass such as ceramics, stone, porcelain, could in the meantime be reliably overcome using automated material selecting methods with opto-electronically controlled sorting apparatus.
For the purpose of sorting colors and for detecting foreign material, methods of contactless measurement by means of infrared or RGB sensors are utilized most of the time. These sensors induce the removal of undesired foreign material from the scrap glass stream or the diversion of colored glass into fractions intended therefore by means of blowing or aspirating nozzles, using therefore the recorded degree of light transmission or absorption of the light directed onto the scrap glass stream.
An issue that heretofore was of minor importance in glass scrap but is gaining prominence lately is the detection of special glass in the scrap glass stream. Special glasses refer to glass types specially created for specific applications and having chemical and physical properties strongly differing from those of normal glass (lime-natron glass), more specifically having a substantially higher melting point and also improved thermal properties. They include glass-ceramics, fused silica glass, lead glass as well as temperature and heat-shock resistant technical glasses such as borosilicate glasses.
Not only in the industry, but in private households as well, there is a need for heat-resistant glass having improved thermal properties and being heat-shock resistant as well. While normal glass already undergoes chemical reactions at about 1,400° C. and melts at about 1,500° C., some special glasses are still capable of approximately keeping their chemical structures in these temperature ranges and resist melting. Glass-ceramics and pure fused silica glass even have a melting point ranging from about 1,800 to 2,000° C. Technical glasses such as borosilicate glasses, which find application in electrical engineering and electronics as well as in optics, are found to be very resistant to heat, expansion and heat-shock.
The primary manufacturing process of special glass is similar to that of normal glass except that, depending on the application field, a certain amount of special oxides are added to the glass melt. A boron-silicate glass may contain about 7-13% of B2O3 and 2-7% of Al2O3 as an additive. Depending on the properties desired, either basic oxides (such as sodium, potassium, magnesium, calcium, barium or zinc oxide) or acidic oxides (such as boron trioxide, aluminium trioxide or diphosphorus pentoxide) are added during glass manufacturing, with metals such as copper, chromium, manganese and iron being utilized as the coloring agents. Boron-silicate glass is resistant to chemical substances and to heat and temperature shocks and find therefore application in the chemical industry, in laboratories, as ampoules and vials in the pharmaceutical industry but also in households such as in the form of cookware and light bulb carriers.
The production of glass-ceramics differs from usual glass manufacturing by an additional last method step. Since in normal glass production, the glass does not crystallize, crystallization nucleating agents such as TiO or Zr02 are added to the glass melt in order to cause the structure to crystallize when a molded glass body is heated anew. Accordingly, the material properties of glass-ceramics are similar to those of ceramics. Since the temperature expansion coefficient of glass-ceramics is equal to zero or even negative, this material is particularly suited for loads involving fast high-temperature changes and is for example used in the form of ceramic hot plates in households.
Lead glasses are another issue: although these glasses are very popular because they are highly refractive and can be readily surface-treated, they must imperatively be recycled in special glassworks where they are remolten under controlled conditions for environmental and health reasons.
It is precisely these described resistance properties of special glass, which are highly appreciated in the respective field of application, that cause considerable problems in the process of glass recycling as they prevent it from melting homogeneously into the normal glass in the crucible, thus disturbing the production process and leading to product flaws.
Some attempts have been made to allow for cost-effective online sorting of special glass. These are based on known methods in which the fractions are separated from the collected broken glass and substantially rely on opto-electronic systems which separate these fractions by means of color recognition in the range of visible light. This is achieved in that the piece that is to be singled out from the mixed scrap glass stream is irradiated from radiation sources while being conveyed on a sorting belt or while in free fall and the intensity of the radiation traversing the scrap glass stream or reflected therefrom is received by a detector unit and compared with reference values. An evaluation and control unit in data communication with the detector unit then associates the piece with a respective fraction and causes it to be grasped by pick-ups or to be diverted into predetermined containers by means of compressed air nozzles or aspirating nozzles.
Approaches exist in which the special glass is detected in the wavelength range of visible light by means of color classification means, mostly RGB sensors, which try to recognize a color from thresholds corresponding to already known special glass colors. Since special glasses mostly also have special shades such as violet or honeydew, this detection method allows for recognizing part of the special glass pieces without however providing for reliable detection of special glass. Since the staining guidelines for special glass are not standardized, some special glasses are also manufactured in conventional unimpressive colors such as white and brown so that they cannot be recognized. Glass in shades of brown in particular cannot be distinguished reliably, so that there is high glass loss because of erroneous sorting. A corresponding separating accuracy can only be generated conditionally since detection by color intensity comparison in the visible and infrared range depends inter alia on the thickness and the shape of the glass.
Other known methods for sorting special glass work with X-ray sensors where certain chemical components (e.g., aluminium oxide) of the special glass are excited by an X-ray source. The excited elementary particles or electrons react, emitting energy in the form of light the intensity of which is finally measured and evaluated for detection. The industrial market accepts this method using an X-ray sensor with reservation though since the utilization of X-rays always poses a certain health hazard to persons involved in the surroundings of the line because of the extremely short-waved radiation. Moreover, lines operating according to this method are of quite large construction and are costly throughout. This method does not ensure complete detection of special glasses, in particular not of some borosilicate glasses.
Another known method for sorting special glass works with the fluorescent property of special glass. Glass is thereby irradiated with UV light of a certain wavelength and starts fluorescing in a narrow visible spectral range since the irradiated light is partially absorbed by impurities present in the oxidic glass and is converted into fluorescence radiation. The color of this fluorescence radiation then allows drawing conclusions about the type of special glass. Such a method is known from DE 43 39 822 Cl for example. It has been found however that it is disadvantageous that the scrap glass stream is irradiated with UV light which depends on the special glass type to be sorted. Put another way, this means that the type of special glass contained in the scrap glass stream must be known before sorting in order to perform the radiation with UV light of the right wavelength.
Still another disadvantage is that the fluorescence effect described is typical for impurities contained in the special glass and not for the type of glass itself. Unwanted impurities or microinclusions in the furnace or cubicle material already occur during the manufacturing of glass, so that the fluorescence behavior is difficult to assess. Insofar the problem is that completely different kinds of special glass may have a comparable fluorescence spectrum and the same kind of special glass different fluorescence spectra. Another disadvantage of this method is that, because of the low concentration of impurities, a very strong light source must be utilized to excite them. This requires high energy expenditure and calls for the need of providing an edge filter for protecting the detector unit on the detector side.